I'll post some pictures just so you can see the boat, but you can't really see what I'm talking about in the photos. Looks like a yellowish reflection in the light from a distance, but you can see up close it's more of a yellow scum maybe. Sorry, but I couldn't really get the photos to show it.

I don't have any pics from last weekends cleaning, but I have an 02 supra launch ssv and the gel coat had been neglected by the previous owner. Out lake is very muddy due to being smaller and fed by a river. Long story short my white gel coat was turning brown due to not having any wax on it.
So I did a little research and found that oxalic acid would remove hard water stains from fiberglass and many other materials.
So further reading I found that a product I use often called "bar keepers friend" had a small snout of this acid. This stuff is in a bottle similar to comet cleaner but gold and blue in color. I took and made a paste with the powder and tested under the swim platform. Came clean with light rag coverage within 20. Seconds.
So I then took one 20-24 oz bottle and mixed it with 3 gal warm water to dissolve the powder in the water.
I then used a car wash mitt to spread on hill and scrub away the grime.
I rinsed thoroughly after applying and letting stand 3-5 min. Rinsed 1 more time then scrubbed with a clean wet towel to remove residue then rinsed, dried and used 6 coats of griots carnauba show wax.

Now it looks like a fresh new boat will post a few pics when I get back to the lake house this weekend.

JJV's is what we use for detailing boats in our shop. They say spray on and rinse off, which works pretty well, I'll also take a rag to it just to aid the cleaning and spread it out without using so much. Really awesome stuff it takes the yellow right off. It doesn't hurt the trailer paint, nor will it wreck your decals.

I had major yellow stains on my old Tige. I spent a whole day on the first side wet sanding and polishing to a nice white shine again. Then I spent a half hour on the other side with some Works toilet bowl cleaner and got the same result.

FSR sold at West Marine does great as well. Blue colored gel, wipe it on, let it sit a few seconds and then wash it with a water hose. For heavy yellow stains, you can scrub with a towel. It will be bright white when you are done.

Be sure after you use any products like this to wax it heavily. The cleaner opens the pores of the fiberglass and you want to seal them back up or it will quickly discolor again.